1. Field of the Invention
This invention relates to a common-rail fuel injection system for an engine. This invention also relates to a method in a common-rail fuel injection system.
2. Description of the Prior Art
Common-rail fuel injection systems for diesel engines are disclosed in various documents such as Japanese published unexamined patent application 62-258160, Japanese published unexamined patent application 2-176158, European published patent application 0307947-A2, U.S. Pat. No. 4,777,921, and U.S. Pat. No. 4,940,034.
The common-rail fuel injection systems include a high pressure tubing which forms a pressure accumulator referred to as "a common rail". The fuel injection systems of this type also include high pressure fuel supply pumps for feeding high pressure fuel to the common rail, and solenoid valves for selectively allowing the high pressure fuel to flow from the common rail through injectors into engine cylinders. In general, the pressure of fuel in the common rail is controlled for accurate adjustment of the rate of the fuel injection into the engine cylinders.
The high pressure fuel supply pumps in the common-rail fuel injection system include pumping chambers, and movable plungers partially defining the pumping chambers respectively. The plungers are driven by the engine through a suitable mechanism. The drive of the plungers pressurizes fuel in the pumping chambers, forcing the fuel from the pumping chambers into the common rail. In general, spill or relief solenoid valves are connected to the pumping chambers respectively. Closing and opening the relief solenoid valves enables and disables pumping the fuel from the pumping chambers into the common rail. Thus, the rate of fuel supply to the common rail is adjusted by controlling the relief solenoid valves.
In general, the relief solenoid valves are of the normally-open type. The valve members of the relief solenoid valves are designed so that they will be urged by the pressure in the pumping chambers toward their closed positions. When a high pressure pump plunger is required to drive the fuel into the common rail, the related relief solenoid valve is energized to move its valve member to a closed position so that the fuel supply from the pumping chamber to the common rail is enabled. Then, the valve member is held in the closed position by a resulting high pressure in the pumping chamber, and the relief solenoid valve can be de-energized to save electric power. The rate of fuel supply to the common rail is adjusted by controlling the timing of energizing the relief solenoid valve, that is, the timing of closing the relief solenoid valve.
In general, the high pressure fuel supply pumps are designed so that when the relief solenoid valves are open, fuel can be fed to the pumping chambers from a low pressure side or a fuel reservoir through the relief solenoid valves. Specifically, after the fuel supply to the common rail from the pumping chamber ends, the related high pressure pump plunger moves in the direction of expanding the pumping chamber so that the pressure in the pumping chamber drops and thus the relief solenoid valve opens. It should be noted that the relief solenoid valve is de-energized a given short time after the start of the energization thereof. When the relief solenoid valve opens, fuel starts to be drawn into the pumping chamber from the low pressure side through the relief solenoid valve.
In such a prior art common-rail fuel injection system, when the energizing winding of a relief solenoid valve breaks, the relief solenoid valve remains de-energized and continues to be open. In this case, the related high pressure supply pump remains disabled, and the fuel supply from the high pressure supply pump to the common rail continues to be unexecuted. On the other hand, when a short circuit occurs so that a relief solenoid valve is continuously energized, the relief solenoid valve continues to be closed. In this case, the fuel feed to the related pumping chamber from the low pressure side remains inhibited, and thus the fuel supply from the high pressure supply pump to the common rail continues to be unexecuted. In both of the above-mentioned two cases, the continuous unexecution of the fuel supply from the high pressure pump to the common rail tends to cause some problem in the control of the pressure of fuel in the common rail. When the valve member of a relief solenoid valve mechanically sticks at its closed or open position, a similar problem occurs.
In cases where the pressure of fuel in the common rail is maintained at a given level by feedback control, such a malfunction of the relief solenoid valve of a high pressure supply pump causes a significantly great increase in the load on the other high pressure supply pump (pumps). The great increase in the load on the other high pressure supply pump is disadvantageous from the standpoint of the life thereof.
U.S. Pat. No. 4,469,065 discloses a fuel pump control system for use in an internal combustion engine having fuel injection valves each driven by a command signal indicative of a required quantity of fuel supplied to the engine. The engine is also equipped with a fuel pump which serves to supply pressurized fuel to the fuel injection valves. In the fuel pump control system of U.S. Pat. No. 4,469,065, at least one abnormality detecting means monitors the injection-valve command signal and a signal indicative of the operating state of a corresponding one of the fuel injection valves. After the levels of the two monitored signals have become out of a predetermined logical relationship, the abnormality detecting means generates an abnormality-indicative signal. The fuel pump is rendered inoperative by the abnormality-indicative signal.